Bidirectional light sheets formed using LEDs have various uses, such as for general lighting of a room. For example, the side facing downward can provide direct lighting, while the side facing toward the ceiling can provide diffused lighting and create an aesthetically pleasing appearance. Phosphors can be used with blue LEDs to create any colors, including white light. The light sheet may also be suspended vertically.
A common solid-state overhead lamp is formed by a flat plastic light guide having a roughened surface. High-power packaged LEDs, such as white light LEDs using a phosphor, are optically coupled to the edge(s) of the light guide, and the LED light is reflected internally by the light guide until it exits through the roughened surface. Such a technique produces uneven lighting across the light guide due to absorption by the light guide and the light leakage. If there is no reflector over the top surface of the light guide, the light will exit bidirectionally.
It is also known to distribute many identical medium-power bare LEDs over a first transparent substrate, having a conductor pattern, and bond the bottom LED electrodes to the conductor pattern. Then, a top transparent substrate with a similar conductor pattern is aligned with and laminated over LEDs. Then the top LED electrodes are then bonded to the top conductor pattern. The LEDs all have a reflective layer forming the cathode contact and have a small anode electrode to allow light to exit the anode surface. No light exits the cathode surface since it is completely covered by the reflective layer. If the LEDs are properly arranged with respect to the conductor patterns, and their orientations (anode up or anode down) are alternated, the LEDs may be connected in series by the conductor patterns and emit light through opposite surfaces of the light sheet. Phosphor may be deposited on the external surfaces of the substrates to select the desired emission color, such as white light. Problems with such a device when forming a wide area light sheet are that: 1) it is very difficult to align the LEDs with the conductor pattern; 2) it is very difficult to align the top substrate with the LEDs; 3) the pick-and-placement process is time-consuming; 4) the failure of an LED or its bond creates an open circuit in the series; 5) and, any bending of the light sheet will create a shearing force between the two substrate and delaminate one of the substrates. Such a bidirectional light sheet is disclosed in US Patent Application Publication No. US 2012/0268732.
Additionally, LEDs having a bottom reflective electrode used to redirect the 50% of light generated in the LED junction emitted toward the bottom of the device back toward the upper surface of the LED have significant optical losses at the mirror. Using typical conductive metal mirrors, it is difficult to achieve higher than 80% to 90% reflectivity at the mirror surface, with the rest lost to absorption, and in turn heat. Total internal reflection (TIR) is always a problem in the high refractive index materials used in LED devices, making light extraction structures such as roughening of the exterior surface of the LED a requirement. The presence of a metal mirror electrode exacerbates the TIR problem by introducing a surface which absorbs a portion of a totally internally reflected light ray at every bounce off the mirror.
What is needed is a solid-state bidirectional light sheet and fabrication technique that does not have the drawbacks of the prior art.